The present invention relates to a tunable transducer. Piezoelectric composite transducer assemblies can be adjusted dynamically with an externally applied signal. Mechanical and electrical adjustment enables operation over a wide frequency band.
By way of background, smart materials sense a change in the environment and, using a feedback system, make a useful response with an actuator. Examples of passively smart and actively smart materials have been described in a recent review paper [Newnhan, R. E., et al., J. Am. Ceram. Soc., 74:463-480 (1991)]. Passively smart materials have self repair mechanisms or standby phenomena that enable the material to withstand a sudden change in the environment. Ceramic varistors and positive temperature coefficient (PTC) thermistors are passively smart materials in which the electrical resistance changes reversibly with voltage (varistor) or temperature (thermistor). When struck by lightning, a zinc oxide varistor exhibits a large decrease in its electrical resistance, and the current is shorted to ground. This change in resistance is reversible. PTC thermistors, such as doped barium titanate (BaTiO.sub.3), show a large increase in electrical resistance at the ferroelectric-paraelectric phase transformation (.about.130.degree. C.). The increase in resistance protects circuit elements against large current surges. Varistors and PTC thermistors function as passively smart materials that use standby mechanisms to prevent electrical breakdown.
Actively smart materials are used in automobile suspension systems to provide controlled compliance for the shock absorber system. The TEMS (Toyota Electronic Modulated Suspension) system [Tsuka, H., et al., A New Electronic Controlled Suspension Using Piezoelectric Ceramics, IEEE Workshop on Electronic Applications in Transportation (1990)] uses a piezoelectric sensor to monitor road roughness. The sensor produces a voltage which is amplified in magnitude and altered in phase, and then, applied to a piezoelectric actuator. The actuator produces a hydrostatically enlarged displacement which adjusts the damping force in the shock absorber system. All of these functions, from sensing to hydraulic adjustment, takes place in less than 20 msec.
By introducing a learning function into smart materials, the degree of smartness is upgraded to very smart. A very smart material senses a change in the environment and responds by changing one or more of its property coefficients. Such material can "tune" its sensor and actuator functions in time and space to optimize behavior. With the help of memory elements and a feedback system, a very smart material becomes smarter with age.
The distinction between smart and very smart materials is essentially one between linear and nonlinear properties. This difference can be demonstrated in the behavior of strain with applied electric field in piezoelectric PZT (PbZr.sub.0.5 Ti.sub.0.5 O.sub.3), and electrostrictive PMN (PbMg.sub.0.33 Nb.sub.0.67 O.sub.3) ceramics. In hard PZT, the strain is linearly dependent to the applied electric field. Therefore, the piezoelectric d.sub.33 coefficient, which is equal to the slope of the strain-electric field curve, is constant and cannot be tuned with a bias field. However, PMN ceramics exhibit very large electrostrictive effects in which the strain is proportional to the square of the electric polarization. The nonlinear relation between strain and electric field can be used to tune the d.sub.33 coefficient. In certain modified PMN ceramics, these values range from zero at a zero bias field to 1500 pC/N at a bias field f 4 kV/cm.
The tunable transducer described herein is an example of a very smart material. Silicone rubber, an elastically nonlinear material, has an adjustable elastic modulus, enabling the transducer to be tuned in frequency and acoustic impedance.
Electromechanical acoustic transducers which employ piezoelectric materials operating at resonance are used as fish finders, biomedical scanners, and sonar systems to search for objects of various sizes. These systems are limited in that the resonant frequency of operation, f.sub.r, as well as the mechanical Q.sub.m, are fixed and depend on the geometry of the transducer and its complex stiffness. The scattering power of the target depends on the frequency of the interrogating wave and the mismatch in acoustic impedance. It is maximized when the wavelength is approximately the same size as the object. Objects of various sizes can be identified using the same transducer if the resonant frequency can be changed accordingly. By creating a composite transducer whose resonant frequency and mechanical Q can be tuned over a wide range, the versatility of a transducer and its interrogation capabilities can be vastly improved.
The present invention overcomes certain above-described disadvantages inherent with various apparatuses and methods of the prior art. The invention presents a tunable transducer of a very smart material. For example, silicone rubber, an elastically nonlinear material, has an adjustable elastic modulus enabling the transducer to be tuned in frequency and acoustic impedance.